1. Field of the Invention
This invention relates to an electrophotographic photosensitive member, particularly to an electrophotographic photosensitive member having a low molecular weight organic photoconductive material which can give improved electrophotographic characteristics.
2. Description of the Related Art
In the prior art, as the photoconductive material to be used in electrophotographic photosensitive member, inorganic photoconductive materials such as selenium, cadmium sulfide, zinc oxide have been known. These photoconductive materials, while having a number of advantages such as chargeability to an appropriate potential in a dark place, small dissipation of charges in a dark place or rapid dissipation of charges by photoirradiation, also have various drawbacks. For example, in a selenium type photosensitive member, crystallization progresses readily by such factors as temperature, humidity, dust, pressure, etc., particularly crystallization occurs remarkably when the atmosphere temperature exceeds 40.degree. C., whereby such drawbacks as lowering in chargeability or generation of white speckles in the image are involved. A cadmium sulfide type photosensitive member cannot obtain stable sensitivity under a humid environment and a zinc oxide type photosensitive member requires sensitization effect with a sensitizing dye typically by Rose Bengal. However, such a sensitizing dye is subject to charging deterioration by corona charging or light fading by exposed light, whereby there is the drawback that no stable image can be given for a long term.
On the other hand, various organic photoconductive polymers typically polyvinylcarbazole have been proposed. These polymers, although excellent in film forming characteristic, lightweight, etc., as compared with the inorganic type photoconductive materials as described above, have been practically applied only with much difficulty up to date. This is because no satisfactory film forming characteristic has been obtained yet, and also it is inferior in respect of sensitivity, durability and stability with environmental change as compared with inorganic type photoconductive materials. Also, there have been proposed low molecular weight organic photoconductive materials such as hydrazone compounds as disclosed in U.S. Pat. No. 4,150,987, triarylpyrazoline compounds as disclosed in U.S. Pat. No. 3,837,851 9-styrylanthracene compounds as disclosed in Japanese Patent Laid-open Publications Nos. 94828/1976 and 94829/1976. Such a low molecular weight organic photoconductive material has overcome the drawback of film forming characteristic which has been the problem in the field of organic photoconductive polymers by selecting suitably a binder to be used, but it cannot be said to be satisfactory with respect to sensitivity.
For such reasons as mentioned above, in recent years, a laminated structure having the photosensitive layer separated in function into the charge generation layer and the charge transport layer has been proposed. An electrophotographic photosensitive member having the laminated structure as the photosensitive layer can be improved with respect to sensitivity to visible light, charge retentive force, surface strength, etc. Such electrophotographic photosensitive members are disclosed in, for example, U.S. Pat. Nos. 3,837,851 and 3,871,882.
However, in the electrophotographic photosensitive member by use of a low molecular weight organic photoconductive material of the prior art in the charge transport layer, sensitivity characteristics are not necessarily satisfactory, and also fluctuations at the light portion potential and the dark portion potential are great when repeated charging and exposure are performed. Thus, there remain some points to be improved.